1. Field of the Invention
The present invention relates to the field of imaging, particularly resist imaging, more particularly to thermal resist imaging, and still more particularly laser direct imaging. The invention describes novel positive-acting thermal resist compositions, thermal resist structures, thermal resist processes, and thermal resist systems, as well as novel synthetic procedures for the formation of the resist materials.
2. Background of the Art
The merits of Laser Direct Imaging (LDI) have been recognized for a long time in the printing industry for production of offset printing plates, and also in printed circuit board (PCB) production. LDI offers the potential benefits of better line quality, just in-time processing, improved manufacturing yields, elimination of film costs, and other recognized advantages. In direct imaging methods, the exposure of only the selected areas of the heat-sensitive coating by a suitably focused source of energy effects required changes in the coating composition. See, e.g. U.S. Pat. No. 4,724,465, U.S. Pat. No. 5,641,608 (McDermid), U.S. Pat. No. 5,713,287 (Gelbart), the teachings of which are incorporated herein by reference.
Thermally-sensitive imaging elements are classified as compositions that undergo chemical transformation(s) in response to exposure to, and absorption of, suitable amounts of heat energy. The nature of thermally-induced chemical transformation may be to ablate the composition, or to change the solubility of the composition in a particular developer, or to change tackiness of the surface, or to change the hydrophilicity or the hydrophobicity of the surface of the thermally-sensitive layer. As such, selective heat exposure of predetermined areas (image-wise distribution of heat energy) of a film or layer formed of a thermally sensitive composition has the capability of directly or indirectly producing a suitably imaged pattern of composition which can serve as a resist pattern in PCB fabrication, or in production of lithographic printing plates.
In a manner similar to that with photoresists, heat-sensitive compositions can be positive-working or negative-working. With a positive-working heat-sensitive composition, the selective exposure of the film to an appropriately focused beam that can generate the requisite thermal energy will either (a) ablate the so-exposed composition, in which case the desired graphic pattern is directly produced, and the pattern is represented by the remaining film portions not exposed to the focused heat energy and thus not ablated, or (b) cause the so-exposed composition to become differentially more soluble in a suitable solvent, in which case what is essentially a latent image is produced in the film. This latent image enables the film to be dissolved in the heat-exposed areas and remain insoluble and left behind in the non-heat-exposed areas, providing the desired pattern. With a negative-acting heat-sensitive composition, selective exposure of the film to the appropriately focused beam of the requisite thermal energy causes the so-exposed areas to become differentially less soluble in a suitable developer, such that contact with the developer dissolves away the areas that are not heat-exposed and leaves behind the heat-exposed areas as the desired pattern.
Through use of the heat-sensitive imaging elements, the ability to produce a pattern on a substrate surface by direct imaging without use of a phototool is greatly enhanced because the imaging beam need only be a suitably focused source of required thermal energy, such as can be formed from low-cost solid state lasers, as opposed to the focused source of narrow-band radiation of a particular wavelength required for direct imaging of photoresists. Focused thermal energy sources, such as an infrared laser beam, are inherently better suited for use in commercial-scale operations in terms of expense, life and reliability than the UV beams needed for direct imaging of many photo-sensitive compositions. Moreover, the heat-sensitive compositions, which need undergo only thermally-induced composition change rather than a photo-induced change in components or compositions, should be inherently less complicated than photo-sensitive compositions in direct imaging processes. As such, not only can thermally-sensitive compositions be formulated in the cost-effective manner needed for industrial uses such as PCB fabrication, but their simpler mechanism of operation enables operation in room light or day light and tends to provide or enable generally better shelf stability.
Positive working photosensitive compositions based on novolak-diazoquinone resins are the main imaging material of the computer chip industry (see, e.g. R. R. Dammel, "Diazonaphthoquinone-based Resists", Tutorial text No. 11, SPIE Press, Bellingham. Wash., 1993).
Compositions of light sensitive novolak-diazoquinone resins are also widely used in the printing plate fabrication. The light sensitive diazonaphthoquinone derivatives (DNQ) added to novolak resins (a phenol-formaldehyde condensation polymer) slows down the dissolution of the resin, and the exposed films (with the photoinduced decomposition product of the DNQ) dissolves even somewhat faster than the pure novolak films. A revised molecular mechanism of novolak-DNQ imaging materials was recently published (A. Reiser, Journal of Imaging Science and Technology, Volume 42, Number 1, January/February 1998, p.p. 15-22) and teaches that the basic features of the imaging phenomena in novolak-diazonaphthoquinone compositions is the observed inhibition of dissolution (of the resin), which inhibition is based on the formation of phenolic strings by the interaction of the strong hydrogen acceptor which acts as a solubility inhibitor with the OH groups of the resin. On exposure, the phenolic strings are severed from their anchor by a thermal effect: the Wolff rearrangement, which follows photolysis of the diazoquinone moiety of the inhibitor molecule. This rearrangement is not only very fast, but also highly exothermic (delta H.degree. is at least -66 kcal/mol). The sudden appearance at the location of the solubility inhibitor of a heat pulse of that magnitude causes a temperature spike of not less than about 220.degree. C. At the high temperature that is produced, the phenolic string is severed from its anchor at the DNQ and becomes inactive (dispersed), because it is no longer held together by the inductive effect of the solubility inhibitor.
This model may explain the fact that a wide range of heat sensitive compositions based on novolak resins where different types of inhibitors were incorporated have appeared in patent literature and in commercial announcements. For example, positive working direct laser addressable printing form precursors based on phenolic resins sensitive to UV, visible and/or infra-red radiation were described (see, e.g. U.S. Pat. No. 4,708,925 (Newman, 3M); U.S. Pat. No. 5,372,907 (Haley et. al., Kodak); U.S. Pat. No. 5,491,046 (DeBoer et. al., Kodak)). In U.S. Pat. No. 4,708,925, the phenolic resin dissolution in alkaline solution was decreased by a radiation sensitive onium salt instead of DNQ, with the native solubility of the resin being restored upon photolytic decomposition of the onium salt. The onium salt composition is intrinsically sensitive to UV radiation and can be additionally sensitized to infra-red radiation. U.S. Pat. Nos. 5,372,907 and 5,491,064 utilize direct positive working systems based on a radiation induced decomposition of a latent Bronsted acid to increase the solubility of the resin matrix on imagewise exposure. All three described compositions can be additionally utilized as a negative working system with additional processing after imaging and predevelopment.
WO 97/39894 (Horsell) describes a heat sensitive composition and a process for making lithographic printing plates by laser direct imaging (LDI). The composition is based on a complex of a phenolic resin and a range of inhibitors containing strong hydrogen acceptors, examples of which are nitrogen containing heterocycles and compounds comprise at least one nitrogen atom which is quaternised. Also, a series of carbonyl containing compounds widely known as inhibitors in UV sensitive compositions of phenolic resins were found to be inhibitors for the heat-sensitive composition. This is not surprising in view of the above-mentioned model (A. Reiser). The composition described in WO 97/39894 (Horsell) is believed not be sensitive to UV and visible light, contrary to the above-mentioned compositions based on onium salts or Bronsted acid decomposition.
WO 97/39894 describes radiation absorbing compounds capable of absorbing incident radiation and converting it to heat. The Horsell reference used, for example, dyes selected from following classes, squarilium, cyanine, merocyanine, pyrylium and other classes known to those skilled in the art. Organic and inorganic pigments such as carbon black or phthalocyanine pigments may also be useful as radiation absorbing materials converting the absorbed incident light to heat.
WO99/08879 (Horsell) describes radiation sensitive compositions for coatings in the production of printed circuits and ther electronic parts and for mask production. These coatings are based on phenolic resins with poor mechanical properties typical of that class of resins.
WO99/01795 (Horsell) describes a method for producing a predetermined resist pattern, for example on a printing plate, circuit board or mask. The method is characterized by the use of a novel polymeric material functionalized by groups to render it insoluble in a developer, but with properties such that exposure to radiation renders it soluble in the developer. The groups are not diazide groups as in conventional systems, but are groups that do not release nitrogen on exposure to radiation and have hydrogen bonding capability. Examples of such groups are 2-naphthylsulfonyloxy, 2-thienylsulfonyloxy, dansylozy, p-toluenesulfonyloxy, benzyloxy and n-butylsulfonyloxy.
WO99/21725 (Horsell) describes compositions based on novolak resins in the production of lithographic printing plates. The solubility of the compositions in aqueous developers is increased in heated areas. The composition contains a compound that increases the resistance in non-heated areas of the heat-sensitive composition to development (dissolution) in aqueous developer. The composition comprising compounds inclusive of A) poly(alkylene oxide) units; B) silane units; and C) esters, ethers and amides of polyhydric alcohols.
WO 98/42507 (Kodak Polychrome Graphics) discloses a positive-working heat-sensitive composition that is imageable using an Infrared (IR) radiation laser without the necessity of a post-exposure baking step and without any flooding exposure step. This composition comprises a phenolicresin, an infrared radiation absorbing compound, and a dissolution inhibitor that is non-photosensitive, and which is capable of providing sites for hydrogen bonding with the phenolic moieties of the binder resin.
WO 99/08157 (Kodak Polychrome Graphics) provides an infrared imaging composition that contains two essential ingredients: a non-basic infrared radiation-absorbing material (such as carbon black) and a phenolic resin that is either mixed or reacted with a diazonaphthoquinone derivative. These compositions are useful in a positive-working or negative-working imaging mode, as in lithograohic printing plates.
WO 99/11458 (Kodak Polychrome Graphics) also describes a phenolic-based resin (or other polymers having pendent hydroxy, carboxylic acid, amide, nitric acid or combinations thereon) and an infrared absorbing compound. The imaging layer may also contain a second polymer that has bonded, pendent groups that are 1,2-naphthoquinone diazide, hydroxy, carboxylic acid, sulfonamide, nitrile, etc. The imaging layer may also contain a solubility inhibiting agent, a visible dye, or both.
A wide range of thermally-induced compositions useful as thermographic recording materials have been disclosed in GB 1,245,924 (Agfa), such that the solubility of any given area of the imagable layer in a given solvent can be increased by the heating of the layer by indirect exposure to a short duration high intensity visible light and/or infrared radiation transmitted or reflected from the background areas of a graphic original located in contact with the recording material. The systems described are varied and operate by many different mechanisms and use different developing materials ranging from water to chlorinated organic solvents. Included in the range of compositions disclosed which are aqueous developable are those which comprise a novolak resin. The patent describes that coated films of such resins show increased solubility on heating. The compositions may contain heat absorbing compounds such as carbon black or Milori Blue (C.I. Pigment Blue 27), these materials additionally color the images for their use as a recording medium.
To our knowledge, there are few positive-working heat-sensitive compositions directly related to fabrication of printing circuit boards using LDI which are described in the patent literature. Some compositions of this type are described in U.S. Pat. No. 5, 641,608 (McDermid), but the coating composition on the copper surface of the printing circuit board was scanned previously using a UV laser and only afterward by the infrared laser. Another example of a positive-acting thermo-resist described in this patent does not contain any radiation absorbing material able to convert the infrared light to heat, and it is not clear how the transparent to IR light composition changes its chemistry (acidic hydrolysis of the protecting t-butyloxy carbonyl group) to provide differentiation in the developer solution. The same is not clear regarding the negative-working compositions exposed to infrared beam, where the negative-working compositions do not contain radiation-absorbing material in the coated on copper transparent to IR film.
The advantages of positive-working photoresist applied as a monolayer dry film, versus negative-acting dry resists, in production of printing circuit boards are described in EP 0848290 (Morton) and in T. A. Koes and S. H. Wheeler, IPC Expo 98 Proceedings, S12-3-1.
Examples of heat-sensitive negative-working resist compositions are presented in U.S. Pat. No. 5,512,418 (Ma, Du Pont), the energy needed for polymerization of the imaged areas is in the range of 600 mJ/cm.sup.2, and the time that will be needed for imaging should be very long, and not answering the requirements for high throughputs in the PCB industry.
Polyvinyl acetals polymers, especially polyvinyl butyrals are used in a very wide range of products as binders, due to their excellent film forming and outstanding mechanical characteristics, these polymers are also known as materials with very good resistance to chemical attack. One of the applications of these polymers is in radiation-sensitive compositions, where they serve as binders for negative-working printing plate fabrication, and production of photoresists for the printed circuit board field. The polyvinyl butyrals or polyvinyl formals belong to materials that are not soluble in aqueous developers used in the preparation of printing plates or printing circuit boards. Many different polymers have been proposed for use as binders in negative-working UV-sensitive compositions that provide the required aqueous solubility.
Particular advantages have been achieved for polymers containing hydroxyl or carboxyl groups in the acetal moieties.
These binder polymers are described in U.S. Pat. Nos. 4,665,124; 4,940,646; 5,169,898; 5,169,897; 5,700,619. A photosensitive material containing a resin binder, including a vinylic polymer having phenolic hydroxyl groups and aromatic diazonium salt having only a single diazo group was disclosed in U.S. Pat. No. 4,374,193. Polymers containing p-hydroxybenzal moieties are also described in U.S. Pat. No. 5,792,823 as precursors for polymers used as chemically amplified positive-working resists in microlithography, but the hydroxybenzaldehyde acetal polymers are not themselves used as the composition in the positive-acting resist layer. JP 09,328,519 teaches that similar polyvinyl acetals used as oxygen inhibitors.